The invention relates generally to offshore seismic prospecting and, more particularly, to ocean bottom cables having multiple sensors, such as hydrophones, geophones, and accelerometers.
In one conventional form of seismic surveying, a vessel tows a seismic source, such as an airgun array, that periodically emits acoustic energy into the water to penetrate the seabed. Sensors, such as hydrophones, geophones, and accelerometers housed in sensor units at sensor nodes periodically spaced along the length of an ocean bottom cable (OBC) resting on the seabed, sense acoustic energy reflected off boundaries between layers in geologic formations. Hydrophones detect acoustic pressure variations; geophones and accelerometers, which are both motion sensors, sense particle motion caused by the reflected seismic energy. Signals from these kinds of sensors are used to map the geologic formations. But the motion sensors are particularly sensitive to vibrations in the OBC that can be transmitted along its stress members, which are generally rigid metal cables.
One OBC system uses structures that are less rigid than the metal stress members to uncouple the OBC acoustically from the sensors and reduce the cable noise. The OBC is cut at each sensor node, the ends of its metal stress members terminated in termination blocks. Flexible stress members are connected between each termination block and the nearest sensor unit. The flexible stress members damp vibrations transmitted along the OBC.
One shortcoming of such an OBC is that the metal stress member has to be cut and terminated at each sensor node. Another shortcoming is that the termination blocks are relatively massive and expensive. And the metal stress member is subject to corrosion. Furthermore, because metal is heavy, the length of an OBC with metal stress members is limited.